PLATO: Planet Hunter Selected by ESA

byPaul GilsteronFebruary 20, 2014

Following up on yesterday’s post on Gaia, it seems a good time to discuss PLATO, the European Space Agency’s planet hunting mission, which has just been selected for launch by ESA’s Science Policy Committee. The agency’s Cosmic Vision program has already selected the Euclid mission to study dark energy (launch in 2020) and Solar Orbiter, an interesting attempt to study the solar wind from less than fifty million kilometers. Solar Orbiter will surely return data we’ll want to discuss here in terms of magsails, electric sails and other ways to harness a solar wind about which we have much to learn.

Solar Orbiter’s launch is the closest of the three, scheduled for 2017, with PLATO pegged for 2024, the launch to be from the European spaceport in Kourou (French Guiana) aboard a Soyuz booster. Note that date, because it’s expected, as this BBC story notes, that the ground-based European Extremely Large Telescope (E-ELT) will be operational in Chile by 2024, a reminder that it should be powerful enough, with its 39-meter primary mirror, to carry out studies of planetary atmospheres as determined by targets PLATO can provide.

The mission will operate from the L2 Lagrangian point 1.5 million kilometers from the Earth, from which vantage PLATO will turn 34 individual telescopes and cameras on to a field of view that encompasses half the sky. Up to a million stars will be under investigation, looking for the characteristic lightcurve of a planet transiting the star as seen from Earth. PLATO will also have a strong asteroseismology component, allowing even more precision in characterizing its planetary finds, especially since these will be followed up with ground-based radial velocity observations that will benefit from having data on surface pulsations on target stars.

Thus PLATO’s unravelled acronym: PLAnetary Transits and Oscillations of stars. The ultimate goals are exactly those we’d expect: To discover and characterize relatively nearby planetary systems, detecting Earth-sized planets and ‘super-Earths’ in the habitable zone around solar-type stars while measuring solar oscillations in the host stars. Working at optical wavelengths, PLATO should be able to determine planetary mass with a precision of 10 percent, planetary radius with a precision of 2 percent and stellar age up to 10 percent.

The BBC quotes Don Pollacco (University of Warwick, leader of the PLATO Science Consortium), on how the mission differs from those that have gone before:

“PLATO will be our first attempt to find nearby habitable planets around Sun-like stars that we can actually examine in sufficient detail to look for life. Nearly all the small transiting planets discovered so far have been beyond our technology to characterise. PLATO will be a game-changer, allowing many Earth-like planets to be detected and confirmed and their atmospheres examined for signs of life.”

The contrast is, of course, with Kepler, whose stars — in a magnitude range between 7 and 17 — were faint enough that follow-up studies for the majority of candidates are difficult if not impossible. The interesting Lost in Transits blog, written by a PhD student at the University of Warwick and thus likely connected with Don Pollacco at the same school, points out that Kepler’s wide field and large camera array will be turned to brighter stars (magnitude 4-16) with equipment sufficient to follow up even small Earth-class planets around these stars. The blog also points to the effectiveness of asteroseismology in this work:

This ability to survey bright stars also allows astronomers to perform extremely sensitive measurements of the stars themselves. By using variations in starlight caused by ripples on the star’s surface, astronomers can accurately pin down not only the size of the star but also the age of the star system. This means, not only can Plato find exoplanets around bright stars, but it can also determine the size and age of many of these planets to a precision only previously dreamed of.

Ahead for PLATO are further refinements and finalization of the design, along with selection of an industrial prime contractor and, within the next two years, the final adoption of the mission, but the support given by the ESA Science Policy Committee, unanimous, strengthens our hopes that PLATO will fly. The fudge factor in that statement simply reflects the disappointments we’ve seen on both sides of the Atlantic with missions like SIM (Space Interferometry Mission) and the Darwin astronomical interferometer all showing promise only to face cancellation. Budgetary realities are always to be reckoned with, but PLATO’s selection is welcome news indeed.

I used to be a great fan of TPF/Darwin. I’m no longer so sure. These were very expensive machines based on large nulling interferometers. If compact star systems are so common as Kepler indicates, would they see anything at all ? Wouldn’t the nulling zone include the planets ?
I guess that depends on how many stars TPF/Darwin would have observed (50-100 ?).
Intuitively, it seems better to have cheaper missions first to survey what will then be observable with a more complicated machine.
Same for some of the coronograph missions I’ve seen, although they are much cheaper than TPF/Darwin and maybe they could be a cheaper survey I just mentioned.

Out of the three ESE’s M-class missions the most interesting from interstellar point of view should be Solar Orbiter. It seems to me that it will be the fastest spacecraft ever build by humans. Small step towards truly interstellar speeds.

Jim, this is from the Lost in Transits blog — the author of same, I’ve learned, does indeed work with Don Pollacco, leader of the PLATO science consortium:

“The Transiting Exoplanet Survey Satellite (TESS), to launch in 2017, seems superficially to be a similar mission to Plato. It will potentially discover hundreds of planets before Plato even gets off the ground in 2024. However, the limited sensitivity of its cameras mean it is completely blind to Earth-like worlds around sun-like stars. Astroseismology is also off-limits for TESS, meaning the size of any worlds it does discover will be highly uncertain. Unlike Plato, it will also move between patches of sky every 30 days, allowing only hot, short-period planets to be found.”

I haven’t completed reading the PLATO 2.0 mission paper yet but from what I’ve read so far PLATO will be a much more capable and powerful instrument then TESS. It should be. The projected cost will be more then $800 million U.S. as compared to $200 million U.S. for TESS.

I think TESS might be a little more optimized for the near-infrared as finding transiting planets orbiting nearby (though visually faint) M-dwarfs will be one of its key goals. As such TESS may provide a target list of transiting planets for the mainly infrared JWST. Though I suppose PLATO could do that as well even if it does launch 6 years after the JWST.

I’m happy that more then one all-sky transit search missions are being planned. In Astronomy it’s great to have multiple observations by different instruments if possible. I see TESS and PLATO as logical successors to the Kepler mission.

Pollaco is the Lost In Transits author’s PhD supervisor . He works predominantly on finding targets in the WASP transit data.
TESS is designed to see and characterise rocky terrestrial, short period ” super earth” style planets as opposed to “mini Neptunes ” ,circling nearby ,”bright” M stars. Its viewing period is at max 25 -50 days which means any such planets would only be within the “habitable” zone of late, small, M stars. The thrust of the mission is to find and characterise terrestrial planets , as opposed to finding exo Earths in a HBZ . The short viewing periods ( it needs three observed transits to confirm a finding) are the limiting factor and what make PLATO so different ,as it has viewing periods of up to three years which allow for discovery of Earth analogues in HBZ zones of bright ( and nearby) stars. The nearby bit is crucial too as this allows accurate mass determination by later ground based telescope RV analysis, something Kepler with its generally dim and distant stars, didn’t.
The Kepler K2 Mission is ironically similar to TESS. It will view fields of bright stars ,for up to 75 days which presumably means three confirmed transits would be 25 days each. One of the PhD students involved , Tom Barclay,also has a blog and confirmed to me that their drive now is to find Terrestrial planets round nearby M stars. Sound familiar? They’ve just proved the new process works by “rediscovering” a known exoplanet ( from WASP ironically) and submitted the data to NASA as part of the review process for new funding. ( they are currently on reserves left over from the original mission.) They intend to start in April regardless and will continue for two years if granted new funding, hopefully some time in May.

Personally I was a little disappointed that EChO (Exoplanet Characterisation Observatory) wasn’t picked. PLATO sounds like a fine mission but I can’t help think that it is going to mean another 10–20 years of only planet finding, not planet characterisation. Irrespective of the publicity put out by ESA, PLATO is not going to be able to truly characterise a planet just from its diameter, mass etc. To do that we really need to understand their atmospheres. The quote from the Lost in Transits blog talks about Earth-like worlds but that’s not strictly true – PLATO will find Earth-mass worlds, but not necessarily Earth-like even if they are in the ‘habitable zone’. At some point we are really going to have to follow-up on them and study and understand their atmospheres to really get an inkling of what these worlds are like. It’s all well and good discovering thousands of worlds, but we don’t really know what any of them are truly like yet. Hopefully after the next round of finding planets around bright and generally closer stars (missions and projects like PLATO, TESS, CHEOPS and the Next Generation Transit Survey) our attention will quickly turn towards fully characterising them.

Dear Keith. I agree with your sentiment . Others did too, there were several supportive articles published by high profile scientists in the run in to the M3 selection . The fact remains though that the a lack of an Earth size planet in the HBZ of a sun like star is still a gap in the accumulated exoplanetary data to date that warrants further cataloging . With PLATO there will be nearly twenty years of near uninterrupted coverage.
Really we needed BOTH missions. Characterising Earth size planets however is notoriously difficult , given the small depth of their atmospheres ,and may well be impossible even longer term for all but the closest stars . ECHO couldn’t have done it. If the WFIRST-AFTA mission gets through the Byzantine Financial approval system around the time of the next US elections , it will offer excellent characterisation through direct imaging ,although for Jupiter size ( and just maybe some bigger super earths) only. The JWST will obviously contribute too , time constraints allowing and CHEOPS will do its bit as well.

In relation to Enzo’s comments above ,they are more relevant to direct imaging than transits or even true interferometry . The key to direct imaging, especially for planets close to there parent stars is the “inner working angle ” which is in effect how small the point spread function of the star can be made , thus leaving any planet outside of it and visible . Remember , both star and planet are only point light sources, NOT spatially resolved which would take enormous telescopes or intrpeferometers with huge baselines and belongs to the distant future. The secret to direct imaging is the coronograph that occludes the central star. There are numerous types , with the best being a combination of a mask ( direct light blocker) and a nulling interferometer. To see an exo earth at 1 AU , a space telescope is required to do two things. It firstly needs a small IWA or inner working angle or angular resolution. The bit that keeps the star blocking effect away from the planet ( 2 times wavelength divided by telescope aperture is the value accepted) . The second is the contrast reduction I.e the coronograph’s ability to reduce the unneeded starlight so it doesn’t swamp the planets reflected light . For an exo earth this needs to be a factor of 10 to the power 10 or 10 billion times. Sounds impressive but several coronographs have done this in the lab, having been tested to a high level in the JPLs high contrast imaging testbed. The biggest problem is in combining the two. Even the NRO 2.4 m mirror proposed for WFIRST-AFTA could achieve this . At present a 10 to the power 9 reduction is being realistic to keep,cost down. ( this level of contrast will see exo Jupiters and Neptunes at 3-5 Au, so still useful in mapping out the outer regions of solar systems that RV and transits have neglected to date) .The smaller the mirror ,however, the more stable the telescope pointing requirement though ( bearing in mind this might need to be for hours) and that is only possible with additional top grade adaptive optics. These take the form of deformable secondary mirrors in the telescopes optical pathway and until recently these were expensive. The new MEM ( microelectromechanical ) systems from Boston are now becoming available at reasonable cost and will on doubt be in space telescopes of the future. NASA have good data on mirrors up to 4 m because these are the largest monolithic type that can fit into current launch vehicles ( ATLAS 5) without being folded up. Segmented mirrors such as in jWST are more expensive , untried as yet and the coronographs that can be used with them to date are limited. Direct imaging is excellent for spectroscopy and characterisation. Frustrating as it is, in the current financial climate , if I was NASA, I would be pushing for a 4 m mirror and forgetting about bigger plans for a while, even if SLS happens which is far from certain. In terms of external occulters, the research to date is sadly limited to small models and then scaled up. These seem to be effective and can be reduced to as small as 25 metres if combined with a coronograph , and much nearer to the parent telescope. It means an extra $200 million dollar launch though plus on board propellant as well as time moving into position . JWST was designed with a later addition of a star shade in mind though.

Enzo is quite right. NEAT/plus is a great proposal and as much as can be possibly achieved with the ESA M class missions in terms of exoplanets. TPF /Darwin was a great idea too but ahead of its time. Ultimately interferometry holds the best hope of truly characterising exo Earths. They are the only practical device that gets the kind of angular resolution needed and are practical once the precision flying is sorted. Coronographs are useful and WFIRST should be a good proof of concept for them , as I understand they are notoriously hard to build and run outside labs. Unfortunately the 2.4 WFIRST mirror is just too small to visualise anything smaller than 2REarth . Because of its NRO heritage , it can’t be used beyond GEO, ruling out a star shade , which really needs precision flying by both shade and telescope, effectively ruling the concept out for JWST too. No way billions will be spent retrofitting JWST now given the monster cost already. Maybe in the future…. A 4 m mirror is the absolute minimum ,off axis, for exo Earth imaging , but won’t be large enough to win the necessary support of extra galactic astronomers. That needs 8 m plus which is a no no until the furore over the JWST cost overrun dies down and even more importantly , it works! $ 8.5 billion for a device that can’t be serviced , works a million miles away and has 170 moving parts! We should all be nervous come 2018 , if anything goes wrong Congress won’t give any money to NASA till the next millennium. The one good thing about the deteriorating situation in Ukraine is that the US will not want to rely on a contract with Russia to take staff to the IST ( China on the moon too! What would Reagan say?) , hopefully driving SLS and the servicing option.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

Centauri Dreams publishes selected comments on the articles under discussion here. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to your fellow readers, I'm probably not going to run it.